Long fiber reinforcing composite structures are characterized as continuous fiber strands of material impregnated with a thermoplastic resin material, pelletized for easy handling, and molded into shaped articles. The shaped articles, ranging for racks for recreation vehicles to bicycle frames, exhibit good impact strength and stiffness due to unidirectional orientation of the reinforcing fiber strands in the shaped article.
Long fiber reinforcing composite structures suitable for the preparation of shaped articles exhibiting good impact strength and stiffness may be prepared according to U.S. Pat. No. Re. 32,772 to Hawley, incorporated herein by reference. Hawley describes a process wherein a mass of thermoplastic resin is extruded through a stationary impregnation die onto a plurality of continuous lengths of reinforcing fiber strands passing through the die to produce a cylindrical-shaped structure. Simultaneously, the resin material is extruded through the die and the reinforcing fibers are pulled through the die at rates sufficient to cause thorough impregnation of individual fiber strands with resin material. The resulting long fiber reinforcing composite structure, described in U.S. Pat. No. 4,312,917 to Hawley, incorporated herein by reference, may be cut into individual inserts, placed into a two-piece stationary mold connected to an extruder, wherein a thermoplastic resin is heated and injected into the mold to provide a coating of resin on the cylindrical inserts to produce a compound composite structure.
Hawley's process for placing a coating of thermoplastic resin on the long fiber reinforcing composite structure has resulted in increased labor and insufficient production of compound composite structures to meet the requirements of manufactures of reinforced thermoplastic shaped articles prepared therefrom.
In an attempt to overcome the labor intensive process of Hawley, and to enhance the appearance as well as the physical and chemical properties of shaped articles prepared from the long fiber reinforcing composite structure, pellets thereof have been physically mixed with additives materials, and the resulting product molded into shaped articles. Generally, the desired additive materials were cut into pellets, physically mixed with long fiber reinforcing composite structure pellets, and molded into the shaped articles. This physically mixing process has failed to provide suitable molding materials. Due to certain incompatibilities between the two substances, perhaps shape, density and pellet size, uniform physical mixtures of the long fiber reinforcing composite structure and additives have been difficult to prepare. Typically, non-uniform blends prepared from these mixtures have resulted in shaped articles exhibiting localized surface areas containing high or low concentrations of either component.
Another process for incorporating additives into the long fiber reinforcing composite structure prior to the fabrication of shaped articles therefrom requires the addition of the additive to the impregnation thermoplastic resin. However, due to the high temperatures required to melt some of the resins to ensure proper impregnation of the fiber strands, sometimes approaching the degradation temperature of the resins, temperature sensitive additives typically degraded and emitted volatile vapors during the heating and extrusion processes.